Ion implantation has become a standard accepted technology used in doping workpieces such as silicon wafers or glass substrates with impurities in the large scale manufacture of items such as integrated circuits and flat panel displays. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece to form a region of desired conductivity. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of the contamination of the workpiece by airborne particulates.
Conventional ion sources consist of a plasma confinement chamber, which may be formed from graphite, having an inlet aperture for introducing a gas to be ionized into a plasma and an exit aperture through which the plasma is extracted to form the ion beam. The plasma comprises ions desirable for implantation into a workpiece, as well as ions which are not desirable for implantation and which are a by-product of the ionization process. The plasma also includes electrons of varying energies.
One example of an ionizing gas is phosphine (PH.sub.3). When phosphine is exposed to a high energy source, such as high energy electrons or radio frequency (RF) energy, the phosphine can disassociate to form positively charged phosphorous (P.sup.+) ions for doping the workpiece and hydrogen ions. Typically, phosphine is introduced into the plasma confinement chamber and then exposed to the high energy source to produce both phosphorous ions and hydrogen ions. The phosphorous ions and the hydrogen ions are then extracted through the exit aperture into the ion beam. If hydrogen ions in the beam or high energy electrons find their way to the surface of the workpiece, they may be implanted along with the desired ions. If sufficient current densities of hydrogen ions or high energy electrons are present, these ions and electrons may cause an unwanted increase in the temperature of the workpiece that may damage structures such as resists on the surface of the substrate, which are employed to mask regions of the workpiece.
In order to reduce the number of unwanted ions and high energy electrons contained within the ion beam, it is known to provide magnets within the source chamber to separate the ionized plasma The magnets confine undesirable ions and high energy electrons to a region of the source chamber away from the exit aperture and confines the desirable ions and low energy electrons to a region of the source chamber near the exit aperture. Such a magnet arrangement is shown in the applicant's commonly-owned, co-pending U.S. patent application Ser. No. 09/014,472, filed Jan. 28, 1998, entitled Magnetic Filter For Ion Source, now U.S. Pat. No. 6,016,036, issued Jan. 18, 2000, which is incorporated by reference herein as if fully set forth. Other related examples of magnet configurations within an ion source chamber are shown in U.S. Pat. Nos. 4,447,732 and 4,486,665 to Leung et al. The Leung references show a magnetic filter comprised of a plurality of longitudinally extending magnets oriented parallel to each other. The Leung '665 patent also shows a negative ion source having a plasma grid assembly. The plasma grid assembly has a plurality of spaced-apart conductive grid members positioned adjacent the ion extraction zone.
An object of the present invention is to improve upon known ion sources having magnetic filters by forming an ion source having an enhanced magnetic field.